MLT GlobeSCFHS exam preparation (Saudi Arabia) – page 36
749 practice MCQs for the SCFHS medical laboratory exam. Level: Intermediate.
On a flow cytometry CD45 versus side-scatter plot, which population shows bright CD45 with low side scatter?
Lymphocytes are small, agranular and express the brightest CD45. Granulocytes have high side scatter and dimmer CD45; blasts show dim CD45 with low side scatter.
An E. coli from chronic diarrhoea in a child adheres to HEp-2 cells in a 'stacked-brick' pattern. The pathotype is:
Enteroaggregative E. coli shows aggregative (stacked-brick) adherence. EPEC shows localized adherence with attaching-and-effacing lesions.
A stool isolate is H2S positive and resembles Salmonella, but is lysine decarboxylase negative, grows in KCN broth and is ONPG positive. It is most likely:
C. freundii is H2S positive but lysine decarboxylase negative and KCN positive; Salmonella is lysine positive and usually KCN negative. Proteus would be urease and PDA positive.
A carbapenem-resistant K. pneumoniae is mCIM positive. In the EDTA-modified test (eCIM), the zone increases by 7 mm compared with mCIM. This indicates:
EDTA chelates zinc and inhibits metallo-beta-lactamases. An eCIM zone increase of ≥5 mm over mCIM indicates a metallo-enzyme; serine carbapenemases are not inhibited.
A patient has cystitis with alkaline urine and struvite encrustation of the bladder wall. Urine grows a slow-growing, lipophilic diphtheroid that is strongly urease positive within minutes. The most likely organism is:
C. urealyticum is a strong urease producer that makes urine alkaline and causes encrusted cystitis. It grows slowly and can be missed if urine cultures are read only at 24 hours.
A dermatophyte isolate produces teardrop microconidia along the hyphae, a wine-red colony reverse on potato dextrose agar, and is urease negative. The most likely species is:
T. rubrum typically gives a red reverse on potato dextrose agar, teardrop microconidia and a negative urease at 7 days. T. interdigitale (mentagrophytes complex) is usually urease positive within a few days.
A black, yeast-like colony that later becomes fuzzy shows annellides producing clusters of oval conidia at their tips. This dematiaceous organism is most likely:
Exophiala (black yeast) begins yeast-like and forms tapered annellides with conidia collecting at the tip. Alternaria is a rapidly growing woolly mold with large muriform conidia.
A motile, oxidase-negative non-fermenter grows from blood of a patient receiving imipenem. It is DNase positive and lysine decarboxylase positive. Which agent is the usual first choice for testing and treatment?
These features fit Stenotrophomonas maltophilia, which carries a chromosomal metallo-beta-lactamase and is intrinsically resistant to carbapenems. Trimethoprim-sulfamethoxazole is the drug of choice. Aminoglycosides are also intrinsically inactive.
A sheathed microfilaria with a tail tip free of nuclei is found in blood collected at midnight. The species is most likely:
W. bancrofti is sheathed, nocturnally periodic, and has no nuclei in the tail tip. Brugia has two separate terminal nuclei; Loa loa has nuclei to the tail tip and daytime periodicity.
PCR mixes containing dUTP and uracil-N-glycosylase (UNG) are used to:
Amplicons made with dUTP contain uracil. UNG removes uracil from any carried-over product before the new run, so it cannot be amplified, while natural DNA is not affected.
A traveller has positive IgM for both dengue and Zika viruses. Which test best identifies the infecting flavivirus?
Flavivirus antibodies cross-react in ELISA. PRNT measures virus-specific neutralizing antibodies and is used to separate dengue from Zika.
The most common O allele produces no active glycosyltransferase because of:
The common O allele has a single guanine deletion near the start of the gene (exon 6), causing a frameshift and a truncated inactive protein. FUT1 mutations cause the Bombay phenotype, not group O.
A new red cell storage system is evaluated. What is the minimum mean 24-hour post-transfusion red cell recovery required for approval?
At least 75% of transfused stored red cells must remain in circulation 24 hours after transfusion. This criterion is used to set the shelf life of red cells in each anticoagulant or additive solution.
Under AABB standards, a granulocyte concentrate collected by apheresis should contain at least:
At least 75% of granulocyte units must contain ≥ 1.0 × 10^10 granulocytes. The figures 3.0 × 10^11 and 5.5 × 10^10 are platelet content requirements.
A whole blood collection took 18 minutes to finish. Many blood centres would NOT use this unit to prepare:
Slow collections may activate platelets and coagulation. Commonly, units collected over 15 minutes are not used for platelets, and those over 20 minutes are not used for plasma for transfusion.
Dombrock antibodies (anti-Doa and anti-Dob) are best described as:
Dombrock antibodies are usually weak, often found with other antibodies, and typing reagents are scarce, yet they can cause hemolytic transfusion reactions.
The Jr(a−) phenotype, caused by variants of the ABCG2 transporter, is most frequent in people of:
Jr(a−) is found mainly in Japanese and other East Asians. Anti-Jra has caused HDFN and transfusion reactions.
Monoclonal IgM anti-D reagents for patient typing are chosen so that they do NOT agglutinate partial DVI cells. Why?
DVI is the partial D most likely to make anti-D. Typing such patients as D-negative protects them; donor reagents, by contrast, should detect DVI.
A patient heterozygous for a hemoglobin variant has an unreliable HbA1c by ion-exchange HPLC. Which method is least affected by most hemoglobin variants?
Boronate affinity binds the glucose adduct itself, not the charge of the hemoglobin, so most variants do not interfere. Charge-based methods can mis-separate variants. No method is valid if red cell survival is shortened.
A patient has normal anion gap metabolic acidosis. Urine Na + K − Cl (urine anion gap) is clearly negative. This suggests:
A negative urine anion gap indicates high urinary ammonium (as NH4Cl), the normal kidney response to diarrhea. In distal RTA the kidney cannot excrete ammonium, so the urine anion gap is positive.